It is commonly known that partial and heterogeneously catalyzed oxidation of a very wide range of organic compounds using molecular oxygen in the gas phase allows numerous basic chemicals to be obtained. Examples include the conversion of propylene to acrolein and/or acrylic acid (cf., for example, DE-A 23 51 151), the conversion of tert-butanol, isobutene, isobutane, isobutyraldehyde or the methyl ether of tert-butanol to methacrolein and/or methacrylic acid (cf., for example, DE-A 25 26 238, EP-A 92097, EP-A 58927, DE-A 41 32 263, DE-A 41 32 684 and DE-A 40 22 212), the conversion of acrolein to acrylic acid, the conversion of methacrolein to methacrylic acid (cf., for example, DE-A 25 26 238), the conversion of o-xylene, p-xylene or naphthalene to phthalic anhydride (cf., for example, EP-A 522 871) or the corresponding acids, and also the conversion of butadiene to maleic anhydride (cf., for example, DE-A 21 06 796 and DE-A 16 24 921), the conversion of n-butane to maleic anhydride (cf., for example, GB-A 14 64 198 and GB 12 91 354), the conversion of indanes to, for example, anthraquinone (cf., for example, DE-A 20 25 430), the conversion of ethylene to ethylene oxide or of propylene to propylene oxide (cf., for example, DE-B 12 54 137, DE-A 21 59 346, EP-A 372 972, WO 89/0710, DE-A 43 11 608 and Beyer, Lehrbuch der organischen Chemie [Textbook of organic chemistry], 17th edition (1973), Hirzel Verlag, Stuttgart, p. 261), the conversion of propylene and/or acrolein to acrylonitrile (cf., for example, DE-A 23 51 151), the conversion of isobutene and/or methacrolein to methacrylonitrile (i.e. the term partial oxidation in this document shall also include partial ammoxidation, i.e. a partial oxidation in the presence of ammonia), the oxidative dehydrogenation of hydrocarbons (cf., for example, DE-A 23 51 151), the conversion of propane to acrylonitrile or to acrolein and/or acrylic acid (cf., for example, DE-A 10 13 1297, EP-A 1090 684, EP-A 608 838, DE-A 10 04 6672, EP-A 529 853, WO 01/96270 and DE-A 10 02 8582), the conversion of isobutane to methacrolein and/or methacrylic acid, and also the reactions of ethane to give acetic acid, of ethylene to give ethylene oxide, of benzene to give phenol, and also of 1 -butene or 2-butene to give the corresponding butanediols, etc.
The catalysts used are normally in the solid state.
Particularly frequently, the catalysts used are oxide catalysts or are noble metals (e.g. Ag). In addition to oxygen, the catalytically active oxide composition may comprise only one other element or more than one other element (multielement oxide compositions). Particularly frequently, the catalytically active oxide compositions used are those which comprise more than one metallic element, in particular more than one transition metal. In this case, reference is made to multimetal oxide compositions. Typically, multielement oxide compositions are not simple physical mixtures of oxides of the elemental constituents, but rather heterogeneous mixtures of complex poly compounds of these elements.
Usually, heterogeneously catalyzed gas phase partial oxidations, in particular those mentioned above, are carried out at elevated temperature (generally a few hundred ° C., typically from 100 to 600° C.).
Since most heterogeneously catalyzed gas phase partial oxidations proceed highly exothermically, for reasons of heat removal, they are appropriately carried out frequently in a fluidized bed or in isothermal fixed bed reactors where they are disposed in a reaction chamber around which a heat exchange medium is passed for the purpose of indirect heat exchange (for example, the catalyst bed may be disposed as a fixed bed in the catalyst tubes of a tube bundle reactor around which a salt melt is conducted for heat removal).
However, heterogeneously catalyzed gas phase partial oxidations may in principle also be carried out in catalyst beds disposed in adiabatic reactors.
It is known that the working pressure (absolute pressure) in heterogeneously catalyzed gas phase partial oxidations may either be below 1 bar, be 1 bar or be above 1 bar. In general, it is from 1 to 10 bar, usually from 1 to 3 bar.
The target conversion is effected during the residence time of the reaction gas mixture in the catalyst charge through which it is passed.
Owing to the generally marked exothermic character of the usually heterogeneously catalyzed gas phase partial oxidations of organic compounds with molecular oxygen, the reaction partners are typically diluted with a gas which is substantially inert under the conditions of the catalytic partial oxidation in the gas phase and is capable of absorbing heat of reaction released with its heat capacity.
One of the most frequently used inert diluent gases is molecular nitrogen which is used automatically when the oxygen source used for the heterogeneously catalyzed gas phase partial oxidation is air.
Owing to its general availability, another inert diluent gas which is used in many cases is steam. Both nitrogen and steam are additionally, in an advantageous manner, uncombustible inert diluent gases.
In many cases, cycle gas is also used as an inert diluent gas (cf., for example, EP-A 1180508). Cycle gas refers to the residual gas which remains after a one-stage or multistage (in the multistage heterogeneously catalyzed gas phase partial oxidation of organic compounds, the gas phase partial oxidation, in contrast to the one-stage heterogeneously catalyzed gas phase partial oxidation, is carried out not in one, but rather in at least two, reactors connected in series (which can merge into one another seamlessly in a common casing), in which case oxidant can be supplemented between successive reactors; multiple stages are employed especially when the partial oxidation proceeds in successive steps; in these cases, it is frequently appropriate to optimize both the catalyst and the other reaction conditions to the particular reaction step and to carry out the reaction step in a dedicated reactor, in a separate reaction stage; however, it can also be employed if, for reasons of heat removal or for other reasons (cf., for example, DE-A 19902562), the conversion is spread over a plurality of reactors connected in series; an example of a heterogeneously catalyzed gas phase partial oxidation which is frequently carried out in two stages is the partial oxidation of propylene to acrylic acid; in the first reaction stage, the propylene is oxidized to acrolein and, in the second reaction stage, the acrolein to acrylic acid; correspondingly, the preparation of methacrylic acid is usually carried out in two stages starting from isobutene; however, when suitable catalyst charges are used, both aforementioned partial oxidations can also be carried out in one stage (both steps in one reactor) heterogeneously catalyzed gas phase partial oxidation of at least one organic compound when the target product is removed more or less selectively (for example by absorption into a suitable solvent) from the product gas mixture. In general, it consists predominantly of the inert diluent gases used for the partial oxidation, and also of steam typically by-produced in the partial oxidation or added as a diluent gas and carbon oxides formed by undesired complete oxidation. In some cases, it also contains small amounts of oxygen which has not been consumed in the partial oxidation (residual oxygen) and/or unconverted organic starting compounds.
The steam formed as a by-product ensures in most cases that the partial oxidation proceeds without significant changes in volume of the reaction gas mixture.
According to the above, the inert diluent gas used in most heterogeneously catalyzed gas phase partial oxidations of organic compounds consists of ≧90% by volume, frequently of ≧95% by volume, of N2, H2O and/or CO2, and thus substantially of uncombustible inert diluent gases.
The inert diluent gases used are firstly helpful in taking up the heat of reaction and secondly ensure safe operation of the heterogeneously catalyzed gas phase partial oxidation of an organic compound by keeping the reaction gas mixture outside the explosion range. In heterogeneously catalyzed gas phase partial oxidations of unsaturated organic compounds, it is frequently also possible to use saturated hydrocarbons, i.e. combustible gases, as inert diluent gases.
It is also known that heterogeneously catalyzed gas phase partial oxidations of at least one organic compound over catalyst beds disposed in at least one oxidation reactor can be operated substantially continuously over prolonged periods over one and the same catalyst beds. The reaction conditions may generally be kept substantially constant.
However, the catalyst bed loses quality in the course of the operating time. In general, the activity in particular of the at least one catalyst bed deteriorates. This is disadvantageous in particular because the reactant conversion is thus reduced with increasing operating time of the at least one catalyst bed under otherwise constant operating conditions, which reduces the possible space-time yield.
EP-A 990 636 and EP-A 11 06 598 attempt to take into account the aforementioned development in the long-term operation of a heterogeneously catalyzed gas phase partial oxidation of at least one organic compound over one and the same catalyst bed by gradually increasing the temperature of the catalyst bed in the course of the operating time under otherwise substantially constant operating conditions, in order to substantially retain the partial conversion on single pass of the reaction gas mixture through the at least one catalyst bed.
A disadvantage of the procedure recommended in EP-A 99 636 and in EP-A 11 06 598 is that, with increasing increase in the temperature of the catalyst bed, its aging process is generally accelerated, which is why the catalyst bed is typically fully exchanged on attainment of the maximum value of the temperature of the catalyst bed.
EP-A 614 872 and DE-A 10 35 0822 recommend delaying the necessity of the full catalyst exchange by regenerating the catalyst bed from time to time (for example conducting a hot mixture of molecular oxygen and inert gas through the catalyst bed from time to time). However, a disadvantage of this procedure is that it entails an interruption in the production over a prolonged period.
DE-A 10 23 2748 recommends, as a compromise solution, instead of fully exchanging the catalyst bed, only replacing a portion thereof with a fresh catalyst charge.
A disadvantage of this proposal is that a partial change of the catalyst bed is also associated with significant cost and inconvenience.
As an approach to a solution, it has also already been proposed in principle to provide a more extensive catalyst bed I accommodated, in a comparatively costly and inconvenient manner, in an isothermal reactor and a smaller catalyst bed II accommodated, in a comparatively less costly and inconvenient manner, in an adiabatic reactor, with the aim that the adiabatic reactor begins to provide assistance where the isothermal reactor by itself no longer achieves the highest conversion, in order thus to delay to a maximum the exchange of the catalyst bed in the isothermal reactor.
However, a disadvantage of this procedure is that it requires an increased number of reactors and thus increased investment.